Scanning force microscopes (SFM) were in developed in 1986 by Binnig et al. (compare: Binnig, G. et al., PhysRev Letters, 1986, Vol. 56(9), p. 930-933) for imaging nonconducting surface with atomic resolution. They have since become a widely used tool in the semi-conductor industry, biological research and surface science. The first SFM was basically a thin metal foil acting as a cantilever, which was jammed between an STM-tip and the sample surface. Since the cantilever was a conducting metal, it become possible to measure the surface corrugation of non-conducting samples by monitoring how the foremost tip of the cantilever pointing towards the sample was deflected while moving across the sample surface on the basis of a tunneling current between the cantilever and a probing tip according to the scanning tunneling microscopy principle. Today, the registration of a laser's deflection from the back of the cantilever on a segmented photodiode is commonly used for this task (compare: Meyer, G. et al., Physics Letters, 1988, Vol. 53, p. 1045-1047).
Just as Binnig and Rohrer were originally interested in doing local spectroscopy on superconductors while developing the scanning tunneling microscope (STM) in 1981 (compare: Binnig, G. et al., ApplPhys Letters, 1982, Vol. 40, p. 178-180). The SFM was soon applied to local measurements of forces between different materials in vacuum, gaseous atmospheres, and in liquid. For many researches in different fields, the SFM has become an instrument for measuring local force-distance profiles on the atomic and molecular scale. Measurements that have been performed recently were concerned with ligand-receptor binding forces (compare: Florin et al., Science 1994, Vol. 264, p. 415-417), the unfolding and refolding of proteins (compare: Rief et al., Science, 1997, Vol. 276, p. 1109-1112), stretching of DNA as well as monitoring charge migration on semiconductors and conductor/insulator surfaces (compare: Yoo, M. J., et al., Science, 1997, Vol. 276, p. 579-582).
Local measurements of forces between tip and surface suffer from the following problems: 1) drift of the positioning arrangement, generally a piezo (immediately after the piezo has been extended or compressed); 2) hysteresis of the positioning arrangement or piezo; 3) mechanical drift; 4) thermal drift between sensor and sample on time scales ranging from seconds to hours; and 5) general mechanical instability resulting from the fact that the sensors' mechanical “feedback” on the sample is typically realized via a mechanical arm of much larger dimensions and mass than the sensor itself.
These problems can be alleviated to some degree if the force between tip and surface and, therefore, the distance between substrate and sample surface is kept constant, for instance, by keeping the deflection angle of the cantilever constant (constant force mode). This is restricted to cases, though, where the lever (cantilever) is actually in contact with the sample surface and the normal force on the tip is large enough so as to be well distinguishable from any background noise.
A minimal force-level in the range of a hundred pN is generally required to provide a stable feedback control. Many interactions, especially of biological molecules under physiological conditions, are in the range well below 100 pN down to the level of thermally induced fluctuation forces of the cantilever. Presently available instruments are not capable of locally stabilized measurements at well-defined distances from the sample in this important force range of thermally fluctuating sensors (few pN).
Furthermore, data often need to be sampled locally over time periods of seconds to hours. Stability problems (as enumerated above) of instruments available to date ultimately render such measurements impossible.